"It is, however, equally clear that a general income tax is an excise
tax laid upon persons or corporations with respect to their income:
that is, a person or a corporation is selected out from the mass of
the community by reason of the income possessed by him or it...

"This is brought out clearly by this court in Brushaber v. Union
Pacific Railroad Co.,
240 U.S. 1, and Stanton v. Baltic Mining Co.,
240 U.S. 103. In the former case it was pointed out that the
all-embracing power of taxation conferred upon Congress by the Constitution
included two great classes, one indirect taxes or excises, and the other
direct taxes, and that of apportionment with regard to direct taxes.
It was held that the income tax in its nature is an excise; that is,
it is a tax upon a person measured by his income...It was further held
that the effect of the Sixteenth Amendment was not to change the nature
of this tax or to take it out of the class of excises to which it belonged,
but merely to make it impossible by any sort of reasoning thereafter
to treat it as a direct tax because of the sources from which the income
was derived."

[Brief for
the United States at 14-15 in Peck & Co. v. Lowe,
247 U.S. 165 (1917). Not in the ruling itself]